The present invention relates to a comminuting apparatus and method, and more particularly to comminution in a mill of the re-entrant circulating stream type commonly referred to as a "Micronizer". The Micronizer is the oldest and most widely used of the re-entrant circulating stream grinding mills, and is described in detail in U.S. Pat. No. 2,032,827 issued Mar. 3, 1936. The basic mill includes a vortex chamber comprising an annular peripheral wall closed by two opposed lateral walls. In its preferred form, the vortex chamber is formed so that the axial length of the peripheral wall is only a small fraction of the diameter of the chamber. The peripheral wall is surrounded by a manifold through which high pressure gas is supplied to a plurality of spaced gaseous fluid nozzles angled so that the gaseous jet streams issuing from them create and propel the fluid circulating in the vortex with both a forward and a radial component of movement relative to the axis of the chamber. The material to be comminuted is usually fed into the chamber by a gaseous nozzle and venturi apparatus at a location near the periphery of the chamber. The action of the mill creates a self-classifying effect so that the finished product can be removed through an outlet located at or near the central axis of the mill.
As pointed out in U.S. Pat. No. 2,032,827 and similarly in U.S. Pat. No. 4,018,388, the gaseous jet stream issuing from the nozzles performs two functions. First, it creates a high velocity circulating stream of gaseous fluid and material within the vortex chamber. This circulating stream functions to classify the material by carrying the lighter fractions of material to the central outlet while the heavier particles circulate adjacent to the peripheral wall due to centrifugal force. The second function of the gaseous jet stream issuing from the nozzles is to impart a transverse or radial component of movement to the particles of material thereby causing them to collide with the circulating particles with sufficient force to comminute the material. Accordingly, the relationship between the circulating stream of material and gas and the gaseous jet stream issuing from the nozzles has a significant effect upon the operation of re-entrant circulating stream type comminuting apparatus. U.S. Pat. No. 4,018,388 describes how the introduction of feed material has a loading effect upon the whirling vortex and hence directly effects the comminuting process in a Micronizer type mill. See also copending patent application Ser. No. 904,665 filed May 10, 1978 for comminuting and classifying apparatus of the re-entrant circulating stream jet type, now U.S. Pat. No. 4,189,102. Also, the velocity of particles diverted by the gaseous jet stream significantly affects the comminuting process since momentum is proportional to the square of the velocity. Therefore, the design of the nozzles is important to the operation of the mill.
Heretofore, it has been the commercial practice in re-entrant circulating stream type mills to position the nozzles in the peripheral wall of the vortex chamber so that the gaseous jet stream issuing from them entrains the material circulating adjacent the inner surface of the wall and projects it through the circulating stream of gaseous fluid and material. It is known that the material load in the circulating stream must have some radial depth or else material accelerated by the gaseous jet stream will have little opportunity for impact at its maximum velocity. Thus, the radial depth of the circulating material must be greater than the distance it takes the gaseous jet stream to accelerate the material to its maximum velocity. In any event, maximum velocity will occur after the jet stream has passed through the coarsest material adjacent the inner periphery of the mill. This explains the often observed and published fact that reducing the feed rate (while maintaining other factors constant) increases the fineness until the circulating load becomes of such slight radial extent that further reduction of the feed rate no longer increases fineness.
Most Micronizer type mills use nozzles of uniform cross-sectional area, frequently called abrupt type nozzles. It has been explained in various technical papers that under most operating conditions such nozzles are advantageous over converging-diverging nozzles. The advantage of a nozzle having a bore of uniform cross-sectional area (hereinafter referred to as a tubular nozzle) is that the final expansion of the gaseous jet stream occurs beyond the nozzle causes a suction at its exit end thereby increasing the entraining ability of the gaseous jet stream issuing from the nozzle. Notwithstanding this, the maximum velocity of entrained material does not occur at the exit of the nozzle because of the time it takes to accelerate the material. Yet it is at or closely adjacent to the wall where a large percentage of the larger particles of material are circulating.
Converging-diverging nozzles would appear to be a useful alternative to tubular nozzles because of their ability to generate extremely high velocity gaseous streams. There may be no more concentrated release of energy, except in explosives, than can be obtained from the discharge of a gas at the exit of a nozzle, and converging-diverging nozzles can produce supersonic velocities. The problem with such nozzles in Micronizers is that it is difficult to entrain material into the gaseous streams because expansion takes place in the nozzle. For that reason, tubular nozzles have almost uniformly been used in Micronizers even though they cannot generate gaseous velocities equal to those produced by converging-diverging nozzles.
In order to take advantage of the high velocities generated by converging-diverging nozzles, there have been attempts to configure the peripheral wall, mostly in the form of a "V" or trapezoid, to try to force circulating material into the gaseous jet stream issuing from the converging-diverging nozzles. However, it has been found that any improvement in comminuting is not due to the shape of the peripheral wall as much as to axially restricting the circulating load and hence radially extending it. Because the gaseous stream issuing from a convergent-divergent nozzle becomes turbulent a short distance from its exit, it finally self-loads with material in a manner similar to the way a tubular nozzle aspirates the material. However, no benefit is obtained from the high, even supersonic velocity of the gaseous material issuing from a converging-diverging nozzle.
Another approach is to operate a Micronizer using steam at extremely high pressures and temperatures. So that the Micronizer uses the same quantity of steam, the nozzles are drilled with much smaller diameter bores. Tests were made on 12, 15 and 20-inch diameter Micronizers at pressures up to 1400 lbs. p.s.i. gauge and 900.degree. F. The results were inferior to what is normally obtained in the same size mills at 175 to 200 lbs. p.s.i. gauge and 700.degree. F. The reason is that the gaseous jet stream issuing from the nozzle is so dense and small in diameter that it literally punches through the circulating stream without entraining material. Still further, there is no room for material to enter the gaseous stream as it exits from the nozzle.
From the foregoing, it should be apparent that the problem is to provide a nozzle structure which will accelerate the vented material to the highest possible velocity and still effectively cause the accelerated material to collide with the circulating material.